I\'m currently using Web Essentials 2013 update 4 to provide support for compiling .scss files in Visual Studio.
The tool has worked fine for me in the past however
I had the same problem; in order to help diagnose it I created a minimal .scss file with dummy CSS rules - it generated the same error.
Then I went to my long-time tested and fully working legacy .scss file - it generated the same error. Something is wrong here, I thought.
After tinkering with the issue for a while, somehow it got fixed by closing Visual Studio, reloading and reopening the solution.